(1) Field of the Invention
The present invention relates to compositions and methods for the improvement of fermentations, and more particularly to compositions and methods for the improvement of the rate, yield, maintenance requirements, and/or other parameters of fermentation operations.
(2) Description of the Related Art
The production and processing of foods, feeds, chemicals and pharmaceuticals by fermentation is well known. The metabolic activities of bacteria, yeasts, fungi, and mixtures of these organisms have been used for thousands of years to modify compounds and to produce new compounds. Raising of bread dough, fermentation of beer and wine, curing of olives, pickles and cheese, cleaning of cocoa and coffee, reduction of indigo and retting of flax are all fermentations with long histories.
More recently, fermentations have been used to produce industrial chemicals, such as ethanol, acetone, butanol, organic acids, as well as enzymes, proteins and special carbohydrates. Microorganisms are also used to produce secondary metabolites, such as penicillin, erythromycin, and other antimicrobial substances, which are the basis of modern antibiotics.
Most modern fermentations are carried out in fermentation vessels under more or less sanitary conditions and under more or less rigorous environmental controls. Some pharmaceutical fermentations for example are maintained under strict sanitation and in pure cultures, while other fermentations, such as for wine and spirits, have more relaxed sanitation requirements.
As with any production operation, fermentation economics depend upon factors such as the yield of a desired product per unit of raw material (substrate) utilized, the rate of production of the desired product per unit of volume of the fermentation vessel, the amount of and cost of other nutrients and supplies that are required by the fermentation, and the length of time the fermentation can be operated without shutdown for maintenance and cleaning.
A fermentation operation that has been increasing in importance is the production and recovery of ethyl alcohol (ethanol) for use as fuel. Ethanol that is produced by fermentation for fuel use has been referred to as bioethanol. In commercial bioethanol production, a starch feedstock, typically corn, is hydrolyzed with enzymes such as amylase to convert complex polysaccharides such as starch into simple sugars that can be metabolized by the fermenting microorganism—usually a yeast such as Saccharomyces cerevisiae. The degraded feedstock is then subjected to anaerobic fermentation to convert the sugars to ethanol and carbon dioxide. A reported yield for conversion of corn to ethanol is 2.5 gallons ethanol per bushel (25 kg) of corn. The actual yield is lower than theoretical due to the inefficiency of the process, and any improvement in the efficiency would increase the effective yield. Based on the information in a U.S. Department of Energy Feasibility Assessment, the overall efficiency of the fermentation step is in the range of 80 to 90%.
Because ethanol for fuel use must be produced at a low cost, in most commercial bioethanol production plants the yeasts that are present in the fermenter at the end of a batch fermentation are recovered for recycle back into a new batch of medium. Due to cost restraints, it is impractical to use rigorous sanitation practices in bioethanol production and contaminating microorganisms, notably acid-producing bacteria, sometimes accumulate in the equipment and in the recycled charge of yeast. If the level of these contaminants rises to noticeable levels, they will convert sugars or ethanol into organic acids—thus lowering the yield of ethanol per unit of substrate. This situation usually requires treating the yeasts with antibiotics, or replacing the yeasts and shutting down and sanitizing the fermentation equipment. In plants that operate continuous-flow fermentations, it is not uncommon to see a build up of contaminating organisms in various parts of the lines or fermenters. This condition also requires plant shut down and sanitization and results in a reduction in plant productivity.
It would be useful to provide methods that would reduce the frequency of plant shutdowns for cleaning and sanitizing. It would also be useful if such methods increased the yield, rate, or productivity of the fermenter, or of the entire fermentation plant. Furthermore, it would also be useful it such methods were easy to administer, did not interfere with the operation of the fermentation plant, and were cost effective.
Terpenes are natural constituents of essential oils that are typically found in plants. Terpenes are based on five-carbon isoprene (C5H8) subunits and can optionally include aromatic rings. Many terpenes are hydrocarbons, but oxygen-containing alcohols, aldehydes and ketones, known as terpenoids, are also found. It is not uncommon for terpenes to be present in fermentations, especially in food and beverage fermentations in which natural plant products are used. But the primary purpose for their inclusion, when their addition is purposeful, is for the flavors and aromas that they add. An example of such a use is the addition of hops to beer fermentations.
The essential oil of hops contains the terpenes myrcene and β-pinene and the sesquiterpenes β-caryophyllene and α-humulene. Terpenoids such as linalool and geraniol and esters such as geranyl isobutyrate and methyl dec-4-enoate are also present. Hops are commonly added to hot wort in the brewkettle during wort boiling with the purpose of adding bitterness to the beer, supplying tannins to help precipitate unwanted proteins, improving beer stability due to antibacterial properties, and reducing the surface tension of the wort so that a vigorous boil can be maintained. See, e.g. The essential oil of hops: hop aroma and flavor in hops and beer, at http://www.realbeer.com/hops/aroma, Jun. 7, 2006. In addition, or as an alternative, hop extracts or concentrated hop oil can be added to the beer fermenter, or even to the beer after fermentation is complete. The content of hop oils in beer is not high, however, and is commonly 10 ppm, or less by weight of the finished beer.
Although terpenes are most commonly included in fermentations for flavor and aroma purposes, other uses have been reported. Muramatsu et al., in U.S. Pat. No. 6,974,685, teach a method for the production of prenyl alcohol, itself a mixture of predominantly diterpenes, by Saccharomyces yeast from sugars in the presence of an oil and optionally a terpene and/or a surfactant. If a terpene, such as squalene or tocopherol was used, it was added to the fermentation media in an amount of 0.01% or more, and preferably 1% or more. If a surfactant was used, non-ionic surfactants, such as polyethylene glycol-type surfactants, were preferred.
On the other hand, however, the inclusion of terpenes in fermentation medium has been reported to be undesirable. In U.S. Pat. No. 4,503,079, King et al. teach the benefits of stripping essential oils from citrus molasses in order to improve the ethanol yield during fermentation. Citrus oils, such as d-limonene and mono-cyclic terpenes, were steam stripped from citrus molasses prior to fermentation. The inventors stated that these materials are known to inhibit fermentation and teach that their removal provides an improved fermentable feed stock.
No reports have been found, however, of the beneficial use of terpenes in a fermentation that does not have a terpene as its primary product for any purpose other than as flavorants and/or aromatics.